1. Field of the Invention
The context in which the invention finds application is the conventional 10-speed bicycle, more particularly, the three types of derailleurs and their control cables with which the 10-speed bicycle may be equipped.
A conventional 10-speed bicycle includes two front sprockets that differ from each other in pitch diameter and are mounted for rotation together around the pedal crank axis, five rear sprockets that differ from each other in pitch diameter and are mounted for rotation together around the rear wheel axis, and a closed loop of roller chain that connects together one each of the front and rear sprockets. Different drive ratios are affected between the pedal crank and rear wheel axes depending upon the pitch diameter of the front and rear sprockets that are connected together by the roller chain. The bicycle also includes a front derailleur for pushing a forward portion of the roller chain from one to another of the front sprockets, and a rear derailleur for pushing a rearward portion of the roller chain from one to another of the rear sprockets. The derailleurs directly determine which of the front and rear sprockets will be connected together by the roller chain; the sprocket alignment position of the derailleurs thereby determining the drive ratio that will be effected between the pedal crank and rear wheel axes. Each derailleur is actuated by cable means that extends from the derailleur to a gearshift device that is located somewhere on the bicycle that is within manual reach of the seated cyclist.
The conventional gearshift comprises a support bracket that, most popularly at the present time, is attached to the handlebar stem -the gooseneck-shaped structure that supports the bicycle's handlebar at its center; and includes two levers that are pivotably attached to the support bracket on opposite sides of the handlebar stem. The levers are movable about ninety degrees from an upright position to a rearwardly projecting horizontal position. The cable coming from the front derailleur is attached to one lever and the cable coming from the rear derailleur is attached to the other lever. When the cyclist moves one of the levers the attached cable is moved and the derailleur that is connected at the opposite end of that cable is moved from one sprocket alignment position to another.
A derailleur is a device that pushes or derails the roller chain off one sprocket and guides it onto the next one. But to do this, the roller chain has to be moving around the sprockets, that is, the sprockets have to be rotating and, in addition, the roller chain must not be clinging too tightly to the connected sprockets. This means that a derailleur can be operated only when the bicycle is rolling so that the roller chain is moving around the connected front and rear sprockets, and also, that the cyclist has to either coast or at least not be pedaling hard during the time when he is moving the levers of the gearshift.
"Difficulty in shifting gears" is cited by the United States Consumer Product Safety Commission in its publication: "Fact Sheet No. 10: Bicycles", as one of the prime causes of accidents associated with bicycles.
Cyclists frequently have a problem memorizing the operating sequence and accurately positioning the two levers of the conventional gearshift. The attention and manipulative skill required to effect the desired drive ratio, or to correct one inadvertently obtained, distracts the cyclist's attention from the road ahead and the surrounding traffic. An accident is sometimes the result.
Especially for beginners, it is frequently necessary to glance downward and rearward at the front and rear sprocket clusters to confirm attainment of the desired combination of the front and rear sprockets. An obvious hazard.
On bicycles having spring-biased derailleurs, and especially where the levers of the conventional gearshift are infinitely adjustable within their limited range of movement, it is difficult to center the roller chain on the selected sprockets. If the chain is not perfectly centered on the sprockets it chatters disturbingly. This not only tends to alarm the cyclist, but it requires a further adjustment of the position of one or both levers to correct the condition, thereby further distracting the cyclist's attention from his driving; again setting the stage for an accident.
Often, a change from the highest to the lowest drive ratio has to be made quickly and with the roller chain perfectly centered on the selected sprockets, as when the cyclist arrives at the foot of a steep hill. If a low enough drive ratio is not obtained in time, the bicycle will stall; the derailleurs cannot then be shifted and the cyclist will have to walk the bicycle to the top of the hill. If the low drive ratio was obtained but the roller chain not perfectly centered, the increased pedal pressure normally applied in going up a steep hill and the resultant increase in chain tension, could result in damage to the rear derailleur, particularly to its idler sprockets, which are made of a hard plastic.
A conventional derailleur is herein defined as one having a support bracket that is adapted for attachment to the bicycle's frame near a given cluster of sprockets, a transfer cage that engages the bicycle's roller chain for pushing it off one sprocket and guiding it onto another, and linkage means connecting the transfer cage directly to the support bracket. The linkage means serves to hold the transfer cage in an upright position while at the same time permitting it freedom to move laterally across the sprockets, that is, alternately in opposite directions along an axis that is more or less parallel to the pedal crank or rear wheel axis around which the sprockets rotate. In addition, the linkage means provides a convenient place for attachment of the cable means that actuates the derailleur. The linkage means most commonly comprises a paralleleogram mechanism; the transfer cage and support bracket being secured to opposite parallel bars of the mechanism. The linkage means functions merely to support and transmit a force or motion, and not to provide guidance for the transfer cage for placement of it in any particular sprocket alignment position. The characterizing structure of a conventional derailleur is the presence of linkage means connecting the transfer cage directly to the support bracket; the characterizing and distinguishing mode of operation is that of the transfer cage being moved by and directly in response to the position of the cable means. The position of the cable is determined by a gearshift. The term "gearshift" is used herein in the sense of "means for moving a cable alternately in opposite directions".
Three types of derailleurs fitting the above description are now in wide use: the spring-biased type of derailleur and two types of positively positioned derailleurs, the pull-pull and the push-pull.
The spring-biased derailleur incorporates a spring that impels the transfer cage toward one end of the sprocket cluster. A single cable wire is connected effectively to one side of the transfer cage in opposition to the spring bias and extends tautly therefrom to the conventional gearshift lever that controls that derailleur. Movement of the lever in one direction pulls the cable and the cable pulls the transfer cage into another sprocket alignment position against the resistance of the spring bias. Movement of the lever in the opposite direction slackens the cable thereby permitting the spring bias to return the transfer cage to a previous sprocket alignment position.
The difficulty of precisely centering the transfer cage of the spring-biased type of derailleur in alignment with a given sprocket prompted the Shimano American Corporation to introduce a derailleur having indexing detent means for perfecting the centering of the transfer cage. But to enable the detent means to do its job, it was necessary to eliminate the spring bias so that in the "at rest" position there would be no force impelling the transfer cage in one direction out of the grip of the detent means. This type of derailleur is presently marketed under the tradename "Positron" and is referred to by its manufacturer as being "positively positioned". Initially it was offered in a "pull-pull" version. Later a "push-pull" type appeared on the market.
In the pull-pull type of positively positioned derailleur, what amounts to two cable wires are effectively connected to opposite sides of the transfer cage, the opposite ends of the cables being connected to and extending tautly from opposite sides of the gearshift lever that controls that derailleur. Movement of the lever in one direction pulls one cable and slackens the other, thereby pulling the transfer cage in a first direction toward an alternate sprocket alignment position. Movement of the lever in the opposite direction reverses the roles of the cables so that the transfer cage is pulled back to a previous sprocket alignment position. In practice, a single cable wire is used that is twice the length used in the spring-biased type of derailleur, the cable wire being folded in half so as to provide two half-segments, the midpoint of the cable being fixed to the lever so that the two half-segments extend from opposite sides of the lever, the ends of the single wire being connected, in effect, to opposite sides of the transfer cage.
In the push-pull type of positively positioned derailleur a single cable wire of ordinary length is again used, but it is of a stiffer construction and is necessarily sheathed by a flexible armored casing that serves to guide the cable wire and prevent it from buckling, the cable wire being connected between one side of the gearshift lever and, in effect, to one side of the transfer cage and being used to push the transfer cage as well as to pull it.
In all three instances the transfer cage depends for movement from one sprocket alignment position to another upon the position in which the actuating cable means is placed by the gearshift means. All three types of derailleurs are therefore appropriately within the field of the invention.
The difficulties experienced with the conventional gearshift/derailleur system are perceived as being caused by the conventional two-lever gearshift and not by the derailleurs, at least they are solvable by substituting a semiautomatic gearshift according to the present invention for the conventional gearshift. Others have tended to fault the derailleurs. For example, the Shimano American Corporation which makes both gearshifts and derailleurs, added the Positron derailleurs to its line to correct at least the chattering problem caused by the transfer cage of the spring-biased type of derailleur not being perfectly aligned with the selected sprocket. Others have designed radically new types of derailleurs or complete transmission systems, even resorting to modifications of other parts of the bicycle to make the bicycle compatible with their proposed solution. This shows that the art has looked away from the present solution which is that of cam means for programming the sprocket alignment position of both derailleurs, simultaneously; the cam means being incorporated in a manually operated, single lever gearshift.
This combination of ideas is a significant advance in the art because (1), it provides a gearshift that not only solves all of the problems described but at the same time retains the advantages presently being provided by the conventional gearshift, thereby presenting a serious challenge to the entrenched position of the conventional gearshift and therefore likely to result in the new benefits being made available to the public; and (2), they result in a gearshift that is entirely compatible with the normal mode of operation of a conventional 10-speed bicycle, that is, one having derailleurs. These points are amplified below because they assist in defining the field of the invention and have not previously been given sufficient consideration by prior workers in the field of the invention.